i've done it many times. just like i've told qoncept, a pound less, is a pound less to the weight of the vehicle. but of course, if you're going to do that, might as well take other stuff out: glove compartment, center console, front passenger seat, spare tire and everything that you can do without. then you'll really accumulate a sizable weight worthy of 'taking out'
the cushions themselves are really light, but its possible to take everything out, all the way to the floor, not just the cushions. I saw it from a WRX one of the guys at the dealership owns. He didnt cover it up though so its just a huge whole to the trunk.